What subway do you take to get from Port Authority to East 79th Street?

One option is to just take the E train (of the A-C-E, the blue line) from 42nd Street-Port Authority to 53rd Street-5th Avenue, and then walk the rest of the way.

Or, you can walk through the tunnel that connects the 42nd Street-Port Authority Station to the 42nd Street-Times Square Station. Then, get on the Uptown N or R trains (of the N-R-Q-W, the yellow line) from 42nd Street-Times Square to 59th Street-Lexington Avenue.